1. Field of the Invention
The invention relates generally to motion picture films recorded with photographic (generally referred to as "optical") sound-tracks and to equipment for playing or reproducing them (e.g., motion picture projectors and associated apparatus). More particularly, the invention relates to improvements in the optical sound-track reproduction portions of such equipment. One aspect of the present invention relates to the detection and optional automatic correction of sound-track lateral location errors with respect to the motion picture film projector sound-head. A further aspect of the invention relates to the detection of non-uniform illumination of the slit used to generate the thin beam of light projected onto the film sound-track. A further aspect of the invention relates to the indication of azimuth errors, when the slit is not aligned perpendicular to the direction of film travel. A further aspect of the invention relates to the detection and reduction or suppression of impulse noise (e.g., "ticks" and "pops"), caused by dirt particles, scratches and imperfections in the optical sound-track.
2. BACKGROUND ART
Optical sound-tracks for motion pictures were first demonstrated around the turn of the century, and since the 1930's have been the most common method of applying sound on film. The principle of operation is to have an exciter lamp which illuminates a narrow slit, perpendicular to the direction of film travel. A lens focusses the image of the slit onto the film sound-track, which runs parallel to the direction of film travel, and lies between the picture and the sprocket holes. Behind the film a photocell or solar cell detects the amount of light being passed through the film, and the current or voltage generated by the cell is amplified and sent ultimately to the theatre loudspeaker(s). The transmission of light through the film is varied by either variations in density (an essentially obsolete technique) or by variations of width, where an ideally transparent varying width of sound-track is situated within an ideally opaque surrounding. This latter type of sound-track is known as "variable area".
The first variable area sound-tracks had one fixed edge, with the other edge a distance apart which varied with the required audio modulation. This type of optical sound-track is referred to as "unilateral". The varying clear width causes the required modulation in light transmission received by the cell. It was realized in the late 1930's that errors in light uniformity along the length of the slit could cause distortion components; for example, a fall-off in illumination at that end of the slit that corresponded to peak modulation level could cause significant second-harmonic distortion. In an effort to reduce this effect and other geometric distortion components, the "bilateral" variable area track was introduced. This format has two modulated edges, identical mirror images around a fixed centerline. This techique is immune to constant-slope slit illumination error, but will develop some slight second and third harmonic distortion components under parabolic light error conditions.
A later development, which is now the standard monaural optical sound-track format, is called the "dual-bilateral" (or "double-bilateral" or "duo-bilateral") sound-track. This format has two bilateral elements within the same sound-track area, thus providing further immunity from illumination non-uniformity errors.
In the mid 1970's stereo variable area (SVA) tracks became increasingly popular, in which two independently modulated bilateral sound-tracks are situated side by side in the same area as the normal monaural (mono) variable area track. A two element solar cell is used in this case to provide two independent output signals, one derived from each of the two bilateral sound-tracks.
It will be noted that the mono bilateral sound-track, the mono dual-bilateral sound-track, and the SVA stereo bilateral sound-track all use the basic principle of illumination non-uniformity distoration reduction through signal duplication. In each case, a signal element is mirrored at least once, reducing distortion components; the total light output is then integrated into the photo or solar cell. It can be seen, though, that the integrated total energy received by the cell includes some redundancy. It is the width of the clear area that is the true signal indicator; an integrated signal of total light received by the pick-up cell includes signal components developed by dirt and scratches, and density and illumination dependent distortion components all additive to the signal itself. From time to time, proposals have been made of methods of detecting the variable area track width, as opposed to the integrated light total (for example, U.S. Pat. No. 4,124,784; U.S. Pat. No. 4,355,383; and "The Colortek Optical Stereophonic Sound Film System" by Mosely, et al, Journal S.M.P.T.E., April, 1978, pp. 222-232); most of the proposals intended to take advantage to this art have not been implemented commercially because of the difficulty of projector sound-head modifications, or because of the cost of necessary electronic packages.
The various aspects of the invention described herein comprise simpler mechanisms for taking advantage of the duplicate information provided by the standard bilateral format, whether used in mono bilateral, dual-bilateral or stereo bilateral applications. All the aspects of the invention described utilize a solar cell with more than the normal number of elements, and in most cases require a cell with double the normal number of elements, such that each half of the bilateral sound-track element can be read independently. In this way the intentional redundancy of the mirror image bilateral track structure can provide several useful indicator signals, in addition to the distortion reduction which is currently the only utilized benefit of the bilateral concept.